1. Field of the Invention
This invention relates to delta modulation systems; and it relates, in particular, to a compatible coder and decoder for translating between analog and delta signal formats.
2. Description of the Prior Art
Continuing studies of delta modulation systems center on coders and compatible decoders. Interest is based upon a trend toward expanded use of digital signals in communication systems. Such use makes it desirable to devise coders and decoders that are sufficiently simple to warrant, from both a cost and a signal quality standpoint, their use on a per line basis as distinguished from more limited applications which could be time shared by plural lines and thereby support the high cost of many prior art coding arrangements.
Significant deterrents to low cost, good quality coders have been the need for fast and precise response to analog signal variations in voice and video communication systems. It has also been desirable to produce such response without having the circuits become electrically unstable and break into intermittent or substained oscillations.
Delta modulation coders typically compare an analog input signal in one sampling time with an approximation of a sample of the same signal from a prior sampling time. The coder produces a pulse or no pulse depending upon whether the input analog signal is smaller or larger than the approximated analog signal. The resulting pulse train is the delta modulation signal representing the analog input signal. Various techniques, such as the adaptation of the feedback signal step size to the rate of input analog signal variation, have been employed to avoid problems of slope overload, i.e., the inability of the digital coder to track a rapidly changing analog signal without generating problems of distortion. However, such techniques are often rather costly.
Another important consideration is that the delta pulse train is often produced by a bistable circuit, such as a clocked D-type bistable circuit which is actuated by comparison circuit outputs. However, if the comparator circuit output level is close to the threshold level of the bistable circuit at the actuation time there is an ambiguity in the response time of the bistable circuit that results in width modulation of the bistable circuit output pulses. That modulation is integrated as distortion in the coder feedback and in a receiving demodulator, and the resulting noise makes a soft talker hard to understand.